Heat curable adhesive coating

ABSTRACT

Cohesive flexible curable adhesive coatings formed from a dispersion of first and second powdered curing components, preferably reactive at ambient temperatures, in a solution of a binder, the binder content being not more than 15%, preferably not more than 2.5% by weight of the dried composition. Also claimed are curable compositions having a specific advantageous reactive particle size distribution; specific epoxy-amine adduct curing agents; and specific polyalkylene oxide binders.

This application is a continuation of application Ser. No. 701,401 filed2-14-85 now abandoned.

This invention relates to adhesive compositions and especially toadhesive compositions that may be used with dimensionallyheat-recoverable articles.

A heat-recoverable article is an article the dimensional configurationof which may be made substantially to change when subjected to heattreatment.

Usually these articles recover on heating towards an original shape fromwhich they have previously been deformed but the term "heat-recoverable"as used herein, also includes an article which, on heating adopts a newconfiguration, even if it has not been previously deformed.

In their most common form, such articles comprise a heat-shrinkablesleeve made from a polymeric material exhibiting the property of elasticor plastic memory as described, for example in U.S Pat. Nos. 2,027,9623,086,242 and 3,597,372. As is made clear in for example. U.S Pat. No.2,027,962, the original dimensionally heat-stable form may be atransient form in a continuous process in which, for example an extrudedtube is expanded, whilst hot, to a dimensionally heat-unstable form but,in other applications, a preformed dimensionally heat-stable article isdeformed to a dimensionally heat-unstable form in a separate stage.

In the production of heat-recoverable articles the polymeric materialmay be cross-linked at any stage in the production of the article thatwill enhance the desired dimensional recoverability. One manner ofproducing a heat-recoverable article comprises shaping the polymericmaterial into the desired heat-stable form, subsequently cross-linkingthe polymeric material, heating the article to a temperature above thecrystalline melting point or, for amorphous materials the softeningpoint, as the case may be of the polymer, deforming the article andcooling the article whilst in the deformed state so that the deformedstate of the article is retained. In use since the deformed state of thearticle is heat-unstable application of heat will cause the article toassume its original heat-stable shape.

In other articles as described, for example in British Pat. No.1,440,524, an elastomeric member such as an outer tubular member is heldin a stretched state by a second member, such as an inner tubular memberwhich, upon heating weakens and thus allows the elastomeric member torecover.

Heat-recoverable articles are often used in applications in which theyare required to be coated with an adhesive. In certain applications sucharticles are required to be used with curable adhesives in order thatthe bonds so formed will exhibit good performance at high temperaturesand/or under high loads. One example of such an application is wherehollow heat-recoverable moulded articles having one or more outlets(often called end-caps, boots, transitions or udders) are used toenclose various parts of cable harnesses. Until recently it had not beenpossible to pre-coat such articles with a heat-curable adhesive thatwould cure when the article was recovered because the relatively lowrecovery temperatures of the articles e.g. about 100° to 130° C. and therelatively short recovery times, e.g. up to 2 minutes, were incompatiblewith an adhesive that would have an acceptably long storage life, and soit was necessary to coat the articles with a two-component curableadhesive immediately prior to installation.

Recently, it has been proposed in U.K. Patent Specification No.2,104,800A. the disclosure of which is incorporated herein by referencethat heat-recoverable articles may be coated with a heat-curableadhesive in which the reactive components are in the form of separateparticles that are mixed together. It has been observed that suchadhesives exhibit a synergistic increase in rate of cure when used witha heat-recoverable article which enables them to cure sufficientlyrapidly when the article is recovered and yet have an acceptably longstorage life.

Although the coated articles so formed work well in practice it has beenfound that the functional performance of the adhesives may beconsiderably improved.

According to one aspect, the present invention provides a heat-curableadhesive composition which comprises a plurality of reactive componentsthat will react together to cure the adhesive composition, thecomponents existing separately from one another in the form of solidparticles that are mixed together and which will fuse when heated, theparticles having a size distribution in which not more than 40%preferably not more than 35% and especially not more than 30% by weightof the particles are less than 50 micrometers in size and substantiallyall the particles that is to say at least 90% and especially at least95% by weight, are less than 300 micrometers in size.

Preferably not more than 60% more preferably not more than 50% andespecially not more than 40% by weight of the particles are less than 75micrometers in size and preferably not more than 70%, especially notmore than 60% by weight of the particles are less than 100 micrometersin size. The preferred particle size distributions are those in whichthe particles have a weight average particle size in the range of from75 to 125 micrometers.

It has been observed that the correct choice of particle sizedistribution is important in determining whether or not the adhesivewill exhibit the required high temperature performance properties. Forexample the German Military V.G. Specification No. 95343 part 14 fortesting harness systems, issued by the Bundesamt fur Wehrtechnik undBeschaffung includes a static load test in which the bonds between theharness components are subjected to a 5 kg force load at 100° C. for 4hours without failing or without shearing by more than 5 mm. It had beenconsidered that particulate curable adhesives having the finest particlesize distributions commensurate with adequate storage stability wouldexhibit the best performance on the basis that the finer the particlesthe greater their surface area/volume ratio and the more intimate theirmixing would be, and indeed this belief was borne out when the adhesiveswere tested using conventional lap-shear test methods. However,surprisingly it has been observed that although lap-shear values improveas the adhesive particle size is reduced, such adhesives fail the V.G.static load test completely and that only those adhesives havingrelatively coarse particles according to this aspect of the inventionpass the test. The reason why this is so is not known. Heat-curableadhesives which may be suitable for the present invention include, thosedescribed in British Patent Application No. 2104800.

Preferred reactive components are those which are capable of reactingtogether at ambient temperatures, e.g. when forced into intimate contactby sheat forces or when dissolved in a mutual solvent.

Preferably one of the reactive components is a thermoplastic epoxyresin, for example an epoxy resin based on a bisphenol A or an epoxynovolak resin, in which case the other reactive component may comprise acarboxylic acid, phenolic resin, anhydride or isocyanate or a materialhaving reactive amine groups. Preferably the adhesive is based on athermoplastic epoxy resin as one reactive component and a materialcontaining reactive amine groups as the or at least one other reactivecomponent. The material containing free amine groups may be a polyamide,for example one that is used conventionally as a hot-melt adhesive.These polyamides are characterized by the fact that their amide linkagesare separated by an average of at least fifteen carbon atoms and haveamorphous structures in contrast with the more highly crystalline, fibreforming polyamides such as nylon 6 or nylon 6.6. The polyamidespreferably have an amine number of at least 5. the upper limit for theamine number being determined by the fact that as the amine numberincreases the polyamides become liquid at lower temperatures.

Alternatively or in addition the or at least one material havingreactive amine groups is one based on a polymer that is the same as orsimilar to that on which the epoxy resin is based. For example, andpreferably, the or at least one material containing reactive aminegroups is an adduct of the epoxy resin that is used with a compoundcontaining reactive amine groups, preferably with an aliphatic diamineor triamine and especially with ethylene diamine or ethylene triamine.

It has been found that the use of an epoxy-amine compound adduct as theother reactive component or one of the other reactive components cansignificantly improve the cure rate of the adhesive in relation to itsstorage life, thereby permitting the storage life of the adhesive or thecured properties thereof to be improved or allowing greater freedom toalter the adhesive in order to optimise its other properties. Thus,according to another aspect, the invention provides a heat-curableadhesive composition, which comprises a plurality of reactive componentsthat will react together to cure the adhesive composition, the reactivecomponents existing separately from one another in the form of particleswhich are mixed together and which will fuse when heated, at least oneof the reactive components being a thermoplastic epoxy resin and the orat least one other reactive component being an adduct of the epoxy resinwith a compound containing reactive amine groups.

The adduct may, if desired, be used as the sole reactive component thatcontains reactive amine groups although it is preferred for the adductto be present in combination with at least one other reactive aminegroup such as a thermoplastic polyamide for example to improveflexibility of the adhesive. Preferably the adhesive contains from 30 to90% and especially from 40 to 80% by weight adduct based on the totalweight of the adduct and other reactive amine group containingcomponents.

Chemical curing accelerators may also be present in the adhesive eitherblended with one of the reactive components or as separate particles.Examples of accelerators include dimethylaminopyridine tris(dimethylaminomethyl) phenol, tin octoate, imidazole or imidazolederivatives such as salts, substituted imidazoles or metal complexesthereof.

A number of inert components may be incorporated in the compositions aslong as they do not adversely affect the increase in rate or level ofcure of the adhesive composition. Also it is preferred if they do notadversely affect the storage life of the adhesive.

Inert components that may be incorporated in the adhesive includeplasticisers such as phthalates or rosin esters, thermoplastic orthermosetting polymers, cured or uncured rubbers inorganic materials tomodify the properties of the uncured or cured adhesive such asreinforcing fillers, reinforcing fibres or microspheres, or tackifiersand the like. The other components, whether organic or inorganic, may bein any appropriate physical form, for example they may be in the form ofpowder, flake or fibres, or they may be compounded with one or more ofthe reactive components, and are preferably present in an amount of from5 to 50 percent by weight based on the total weight of the composition.

Electrically conductive fillers. e.g. metal flakes may be incorporatedin the adhesive to provide electrical continuity. e.g. when theadhesives are used to seal parts of an electromagnetically shieldedelectrical wiring harness, as discussed hereinafter.

Preferably the adhesive compositions according to the invention containa polymeric binder in order to prevent the adhesive crumbling off thesurface to which it is applied before it is cured. This is a particularproblem when using particulate adhesives according to the invention thatare applied to heat-recoverable articles since the heat-recoverablearticles, which will usually be in an expanded configuration, aregenerally easily deformable and are usually subjected to various degreesof mechanical abuse in storage. It has surprisingly been found thatuseful degrees of cohesion can be achieved without unacceptablyaffecting the curing performance.

Thus, another aspect of the invention provides, a cohesive flexiblesubstantially solid heat-curable adhesive coating formed from a mixtureof substantially solid particles of a first reactive component withseparate substantially solid particles of at least a second reactivecomponent in a solution of not more than 15% by weight, based on thetotal dry weight of the coating, of a binder, the binder solutionsolvent being substantially a non-solvent for the first and secondreactive components which are present in the dry coating assubstantially unreacted separate particles capable of reacting togetherto effect curing when the coating is heated to the curing temperature.Preferably the binder content will be less than 10% more preferably lessthan 5%. and even more preferably less than 2.5%. and preferably atleast 1% by weight. It will be understood that references to"substantially solid" particles exclude liquid filled microcapsules andare intended to mean particles which are substantially non-flowable (torender the coating storage stable) at the intended maximum storagetemperature.

It is preferable to apply the adhesive onto the article by forming adispersion of the reactive components in a non-solvent, either sprayingor dipcoating the adhesive, in which case the binder should be solublein the non-solvent, and then removing the non-solvent by evaporation.The preferred binders for use with the adhesive are unsubstituted orsubstituted polyalkylene oxides that are soluble in water andwater-soluble polyvinyl acetate polymers having substantially no freehydroxyl groups. The preferred binders are the polyalkylene oxides andespecially polyethylene oxide. It has been found that most solvents withthe exception of water, water based solvents and light aliphaticsolvents such as hexane. solubilise one or other of the reactivecomponents at least to some extent and so are unsuitable, while thosebinders that were considered for use with acceptable non-aqueoussolvents lead to unacceptably poor cured adhesives. When water is usedas a non-solvent for the dispersion the alkylene oxide and vinyl acetatepolymers mentioned above produced adhesives having the best propertiesalthough it is envisaged that some other water-soluble polymers havingsubstantially no free hydroxyl groups may be suitable. Whatever polymeris used for the binder, it is preferred that it be thermoplastic havinga melting or softening point no higher than 20° C. above the highestmelting or softening point of the reactive components and preferably nohigher than the highest melting or softening point of the reactivecomponents, in order to ensure that the binder melts completely when theadhesive is cured. When the adhesive is used in conjunction with aheat-recoverable article, the binder should have a melting or softeningpoint no higher than the recovery temperature of the article andpreferably no higher than 20° C. below the recovery temperature of thearticle, whatever the melting point of the reactive components may be.

Thus according to another aspect, the invention provides a heat-curableadhesive composition which comprises.

(a) a plurality of reactive components that will react together to curethe adhesive composition, the components existing separately from oneanother in the form of solid particles which are

mixed together and which will fuse when heated and

(b) a polymeric binder comprising a substituted or unsubstitutedpolyalkylene oxide that is soluble in water.

Preferably the binder is present in the adhesive compositions only torelatively low levels, for example up to 15% by weight, more preferablyup to 10% by weight and especially up to 5% by weight, based on thetotal weight of the adhesive composition, in order that the binder doesnot adversely affect the cured properties of the adhesive. It has beenfound that even though the quantity of binder used is generallysignificantly below the quantities used in the prior art, the uncuredadhesive does not show any unacceptable tendency to crumble off thearticle during storage. Preferably the binder is present in quantitiesof at least 0.1%. more preferably at least 0.2% and especially at least0.5% by weight, the optimum binder level being about 2% by weight. basedon the total weight of the adhesive composition.

In addition, the use of relatively low quantities of binder reduces theadverse affect that binders generally have on the performance of theadhesives and, in particular, on their high temperature performance.

Preferably the adhesive composition is as described above.

The adhesive compositions according to the invention are particularlyuseful as latent curing adhesives for dimensionally heat-recoverablearticles in view of their good storage life and their relatively rapidcure rate during recovery of the article. In order to maximise the curerate of the adhesive, it is preferred that the or each reactivecomponent has a melting or softening point that is no higher than 20° C.below the recovery temperature of the article. Preferred articles withwhich the adhesives are used include open-ended hollow articles whichare, for example, used for enclosing parts of electrical harness and thelike, although the adhesives may be used with other heat-recoverablearticles and, if desired, with articles that are not heat-recoverable.Such articles, which are often termed end-caps, boots. transitions orudders, depending on the number of outlets, are preferably coated withthe adhesive on their internal surface in the region of the or eachoutlet, but not in the central region. This may be achieved by applyingthe adhesive by a dip-coating method in which the outlet of the articleis dipped in an aqueous dispersion of the adhesive, or if possible adispersion in another non-solvent, and the water or other non-solvent isallowed to evaporate off.

The following Examples illustrate the invention.

EXAMPLE 1 to 3

A heat curable adhesive composition was formed based on the followingcomposition.

    ______________________________________                                                                         Parts                                        COMPO-                           by                                           NENTS                Trade name  weight                                       ______________________________________                                        1         Epoxy resin    Epikote 1001                                                                              100                                                               (Ex Shell)                                           2 (cure agent)                                                                          Polyamide      Macromelt 6071                                                                            25                                                                (Ex Henkel)                                                    Epoxy diamine adduct                                                                         EDA 870 (Ex 75                                                                Anchor Chemi-                                                                 cals)                                                          Aluminosilicate filler     20                                                 Carbon black                2                                       3 (flexibilizer)                                                                        Acid/Ethylene/vinyl                                                                          CXA 2002    10                                                 acetate terpolymer                                                                           (Ex du Pont)                                         4 (binder)                                                                              Polyethylene oxide                                                                           Polyox        2.4                                                             (Ex Union                                                                     Carbide)                                             ______________________________________                                    

Component 2 was formed by melt blending and then grinding on a fanbeatermill at room temperature until the desired particle size distribution isreached by adjusting the grinding conditions. Component 1 was alsoground on a fan beater mill in the same manner as component 2. Component3 was cryogenically ground, and components 1, 2 and 3 were then tumbleblended. Component 4 was dissolved in a quantity of water equal to thetotal weight of the other components to form a 2% solution to which wasadded the powdered components 1, 2 and 3 to form an aqueous dispersionof the complete adhesive.

This dispersion was applied to a standard Raychem 202K153 boot formedfrom Raychem -25 material by a dip-coating method, and the adhesive wasdried in air at room temperature for 6 hours followed by drying undervacuum at room temperature for a further 7 hours. The resulting adhesivethickness was 0.6 to 0.7 mm.

The boot was recovered about a cable and the adhesive joint was testedby means of the static load test at 100° C. specified in the V.G.Specification No. 95343 part 14 as well as by the lap shear testaccording to A.S.T.M. D.1002. In the static load test, the cable andboot are subjected to a 5 kg load at 100° C. for 4 hours and the lengththat the cable had pulled out of the boot or the time to failure of thebond is recorded. For those bonds in which the cable had not been pulledout of boot, the strength of the resulting bond is then measured at roomtemperature. The results are shown in Table I for three adhesives thathad differing particle size distributions. The particle sizedistributions of the adhesives are given in Table II.

                  TABLE I                                                         ______________________________________                                                       100° C. lap                                                                       V.G. static load test                                              shear      (100° C.) strength and pullout               Exam- Particle strength   of passes time to failure of                        ple   size     N(25 mm).sup.-2                                                                          failures                                            ______________________________________                                        1     Coarse   140        5 passes                                                                      mean pullout = 1 mm                                                           mean strength - 453 N                               2     Medium   250        1 pass                                                                        pullout = 1 mm                                                                strength - 360 N                                                              2 failures                                                                    failure time 1.5 hr                                                           and 1 hr                                            3     Fine     300        All fail within 10 minutes                          ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        Weight percentage of particles less than                                      Example                                                                              50 um     75 um   100 um  150 um                                                                              300 um                                 ______________________________________                                        1      25        35      49      72     96                                    2      48        61      81      95    100                                    3      55        81      100     100   100                                    ______________________________________                                    

EXAMPLES 4 to 7

An adhesive composition was prepared according to Example 1 with theexception that the curing agent (component 2) was formed from a blend oftwo polyamides (80 parts Macromelt 6071 and 20 parts Versamid 140), andthat the binder content was varied between 0 and 4%. The adhesive wasapplied to a -25 Raychem moulded part as an aqueous dispersion and driedand then used to form a bond between it and either a cable having aRaychem DR-25 jacket or an adaptor. The room temperature peel strengthwas measured according to V.G. 95343 part 14 Section 2.2 and 2.3. Theresults are shown in table III from which it can be seen that the cablepeel strength increases considerably for small quantities ofpolyethylene oxide binder. In addition, the moulded parts were subjectto a further test before being recovered onto the cable, in which theywere squeezed to 75% of their original diameter in one direction andthen rotated through 120° about their axis before being squeezed again.None of the adhesive was observed to fall off the part in any instancein which the binder level was between 1 and 4%.

EXAMPLES 8 to 10 (comparison)

Examples 5 to 7 were repeated with the exception that the binder was astyrene-isoprene-styrene rubber and the adhesive dispersion was formedin hexane as the non-solvent. The results for the cable peel and adaptorpeel strength are also shown in table III and show that the rubber basedbinder gave significantly inferior performance.

                  TABLE III                                                       ______________________________________                                                    Binder content                                                                            Cable peel                                                                              Adaptor peel                                Example     (weight %)  (N)       (N)                                         ______________________________________                                         4 (comparison)                                                                           0           63        132                                          5          1           112       130                                          6          2           113       110                                          7          4           111       150                                          8 (comparison)                                                                           1           25         20                                          9 (comparison)                                                                           2           40         43                                         10 (comparison)                                                                           4           29         38                                         ______________________________________                                    

EXAMPLES 11 AND 12

In order to demonstrate the improved storage stability and curing of theadhesive formulations that contain an adduct the following two adhesiveformulations were prepared by the method described in Example 1.

    ______________________________________                                                                         Parts                                                                         by                                           Example                                                                              Components                weight                                       ______________________________________                                        11     (1) Epoxy   Epikote 1001      100                                             (2) Cure agent                                                                            Polyamide - Macromelt 6071                                                                      50                                                          adduct - EDA 870  50                                              (3) Flexibiliser                                                                          CXA 2002          20                                       12     (1) Epoxy   Epikote 1001      100                                             (2) Cure agent                                                                            Polyamide Macromelt 6071                                                                        80                                                          Polyamide Versamid 140                                                                          20                                              (3) Flexibiliser                                                                          CXA 2002          20                                       ______________________________________                                    

The minimum viscosity of the adhesives with respect to time weremeasured at 100° C. using a Ferranti-Shirley Viscometer both when freshand after a few weeks storage at ambient temperature.

For Example 11 the minimum viscosity on heating reduced from 900 poisewhen fresh to 464 poise after 10 weeks at 40° C. whereas for Example 12the minimum viscosity increased from 260 poise when fresh to 1125 poiseafter 12 weeks storage at 40° C. indicating that the adduct did not cureto any significant extent on storage whereas the polyamide based onedid.

we claim:
 1. A heat curable-adhesive coating on a substrate made bycoating comprising a mixture of substantially solid particles of a firstreactive component with separate substantially solid particles of atleast a second reactive component, the first and second reactivecomponents being capable of reacting together to effect curing when thecoating is heated to the curing temperature, and the particles having asize distribution in which not more than 40% by weight of the particlesare less than 50 micrometers in size and substantially all the particlesare less than 300 micrometers in size.
 2. A coating according to claim1, further including a polymeric binder which is substantiallyunreactive with the first and second reactive components at least atambient temperatures.
 3. A coating according to claim 2, comprising notmore than 15% by weight of the binder.
 4. A coating according to claim3, comprising 0.5 to 5% by weight of the binder.
 5. A coating accordingto claim 4, comprising not more than 2.5% by weight of the binder.
 6. Acoating according to claim 2, 3, or 4, comprising at least 1% by weightof the binder.
 7. A coating according to claim 2, wherein the particlesof the first reactive component are connected to those of the secondreactive component substantially only by the binder.
 8. A coatingaccording to claim 1, wherein one of the reactive components is athermoplastic epoxy resin.
 9. A coating according to claim 8, whereinthe or at least one other reactive component is a material havingreactive amine groups.
 10. A coating according to claim 8, wherein theor at least one other reactive component is an adduct of the epoxy resinwith a compound containing reactive amine groups.
 11. A coatingaccording to claim 9 or 10, wherein the material having reactive aminegroups is a polyamide.
 12. A coating according to claim 9 or 10, whereinthe material containing reactive amine groups is ethylene diamine orethylene triamine.
 13. A coating according to claim 1, wherein theparticles have a size distribution in which not more than 30% by weightof the particles are less than 50 micrometers in size.
 14. A coatingaccording to claim 2, wherein not more than 60% by weight of theparticles are less than 75 micrometers in size.
 15. A coating accordingto claim 14, wherein not more than 50% by weight of the particles areless than 75 micrometers in size.
 16. A coating according to claim 1,wherein the particles have a weight average particle size in the rangefrom 75 to 125 micrometers.
 17. A coating according to claim 2, whereinthe first and second reactive components are capable of reactingtogether when brought into reactive contact at ambient temperatures. 18.A coating according to claim 2, wherein electrically conductive solidparticles are incorporated to render the cured coating electricallyconductive.
 19. A coating according to claim 2, wherein substrate is adimensionally heat-recoverable article.
 20. A coating according to claim19, wherein the article is a hollow article with at least one opening tothe exterior and has the adhesive coating on the inner surface thereofin the region of an opening.
 21. A coating according to claim 19,wherein a reactive component has a melting or softening point no higherthan 20° C. below the recovery temperature of the article.